Dietary Thyrotropin-Releasing Hormone Stimulates Growth Rate and Increases the Insulin: Glucagon Molar Ratio of Broiler Chickens 1

Abstract

The effects of thyroid manipulation on growth, feed efficiency, and plasma hormone levels were determined in rapidly growing chickens. Beginning at 3 weeks of age, eight broiler cockerels were provided with control feed (CF) or feed containing either 1 ppm of triiodothyronine (T₃), 1 ppm of thyroxine (T₄), 0.3% propylthiouracil (PTU), or 5 ppm of thyrotropin-releasing hormone (TRH) for 3 weeks. Blood samples were taken at 4, 5, and 6 weeks for determination of plasma levels of growth hormone, insulin-like growth factor, T₃, T₄, insulin, glucagon, glucose, and nonesterified fatty acids. Dietary TRH increased (P < 0.05) the growth rate of chickens by 14% when compared with the CF group. Plasma growth hormone levels were reduced (P < 0.05) 65% by dietary T₃ and 33% by treatment with either T₄ or TRH when compared with the CF group. Plasma insulin-like growth factor levels were 16% lower (P < 0.05) in PTU-fed birds than the other treatment groups. Plasma T₃ levels were elevated (P < 0.05) 3-fold by dietary T₃ and 38% by TRH whereas plasma T₃ in the PTU group was 38% below the average of CF birds. Plasma T₄ levels were increased (P < 0.05) by 12-fold in T₄-fed birds, decreased 48% in TRH-fed birds, and nondetectable in birds treated with either T₃ or PTU. Compared with the other treatments, dietary PTU increased (P < 0.01) plasma insulin levels 4.3-fold whereas TRH provided a 2.7-fold increase in plasma insulin. Plasma glucagon levels were 26% higher (P < 0.05) in T₃-fed birds than those fed either T₄ or PTU. These observations indicate that thyroid activity plays an important role in regulating secretion of GH and the pancreatic hormones. Furthermore, our study demonstrates the potential use of TRH as an orally active growth promoter for poultry.